So my choice is Kitty, definetly. For many reasons, but the main ones:

1. She is the star of the storyarc, so deserves to be on this sequel
2. Rogue has had a decent presence on the trilogy, but Kitty only had a decent role on X3
3. Kitty working with the 60's x-men would be really cool, and will bring a very useful power.

I think it would be cool to have Kitty (Page) interacting with the 60s environment... But I'd personally like an Xavier mind swap. I would still open the movie with either Rogue or Kitty in desolate NYC escaping Sentinels and meeting up with Wolverine...